My two bob's worth. Anytime there is a big stable high should be fine, I've crossed in winter in perfect conditions. Can be very windy in Spring. Generally speaking, Jan through to April should be more stable. One consideration I think that is worth making, is the length of daylight. Closer to summer, you will have long hours of daylight. It would be better to commence the flight earlier in the day than later. If, in the extremely unlikely event of a swim, your chances of rescue/survival are a lot better with the luxury of daylight. The guys in Aerocommander that ditched many years ago may have survived if they had more time for help to arrive. Lastly, in a easterly weather pattern, the north east corner can be an isolated area of non or marginal vmc, and there can be weather against the mountains north east of Launceston, so be prepared to go coastal to the Tamar River, pretty much foolproof. You will have a great flight, make the effort to head down the south west as well, the Arthur Ranges, and coastal round the Tasman Peninsula, it's world class, leaves the Cliffs of Mohir for dead.